Abstract
Global spread of anoxia in aquatic ecosystems has become a major issue that may potentially worsen due to global warming. The reconstruction of long-term hypolimnetic anoxia records can be challenging due to lack of valid and easily measurable proxies.The sedimentary Mn/Fe ratio measured by X-ray fluorescence (XRF) is often used as a proxy for past lake redox conditions. Yet the interpretation of this ratio can be problematic when Fe and Mn accumulation is not solely redox driven. We used the varved sediments of Lake Moossee (Switzerland) to examine the partitioning of Fe and Mn in seven fractions by sequential extraction under various oxygen conditions over the last 15,000 years. We combined this data with XRF scans and an independent diagnostic proxy for anoxia given by a hyperspectral imaging (HSI)-inferred record of bacteriopheophytin, to validate the use of the XRF-Mn/Fe ratio as redox proxy.In the 15,000-year long record, Fe was bound to humins and amorphous, crystalline, sulfide and residual forms. Mn was mainly present in carbonate and amorphous forms. Higher erosion, prolonged anoxia, diagenesis and humic matter input affected Fe and Mn accumulation. Under holomixis the XRF-Mn/Fe ratio successfully reflected lake redox conditions. Periods with higher detrital Fe input obscured the applicability of the ratio. During phases of permanent anoxia, intensified early diagenetic processes trapped Mn in the sediments in carbonate, crystalline oxide and humic forms. Our study shows that the single use of the XRF-Mn/Fe ratio is often not conclusive for inferring past lake redox conditions. The application of the XRF-Mn/Fe as a proxy for anoxia requires taking into account the individual lake characteristics and changes in lake environmental conditions, which affect the accumulation of Fe and Mn in the sediments.
Highlights
Oxygen is fundamental in lake systems (Wetzel, 2001), and the abundance or the lack thereof controls many chemical, biological and physical lake processes (Imboden, 1998)
Our study shows that the single use of the X-ray fluorescence (XRF)-Mn/Fe ratio is often not conclusive for inferring past lake redox conditions
We address the following questions: 1) How did changing catchment processes and lake biogeochemical cycles influence net burial rates and partitioning of Fe and Mn in postglacial times? 2) What are the conditions under which XRF-Mn/Fe ratios provide reliable information about past lake water oxygenation? We use a sequential extraction method modified from Hall et al (1996) to measure seven Fe and Mn fractions, combined with scanning XRF elemental data, Total organic carbon (TOC), TIC and an existing record of anoxia inferred from sedimentary pigments (Makri et al, 2020)
Summary
Oxygen is fundamental in lake systems (Wetzel, 2001), and the abundance or the lack thereof controls many chemical, biological and physical lake processes (Imboden, 1998). Oxygen regulates water column and water-sediment interface chemistry through redox processes affecting water quality, biodiversity (Nürnberg, 1995) and ecosystem services in general (United Nations, 2015) Both natural and human factors, e.g. warmer temperatures and excessive nutrient inputs can regulate the establishment of anoxic conditions in lakes (Fang and Stefan, 2009; Foley et al, 2012) mainly by enhancing thermal stratification (Woolway and Merchant, 2019) and stimulating aquatic productivity (Schindler, 2006). Other diagnostic proxies used so far, such as lipid biomarkers (Naeher et al, 2012) or pigments indicative of the presence of sulfur bacteria at the chemocline (Wirth et al, 2013), are difficult to measure Another widely used approach is the Mn/Fe ratio inferred from scanning X-ray fluorescence (XRF) (Friedrich et al, 2014; Naeher et al, 2013), which provides semi-quantitative information about lake oxygenation from seasonal to millennial scales. High Mn/Fe ratios may indicate increased O2 concentrations, and low Mn/Fe ratios may indicate reducing conditions (Boyle, 2001; Mackereth, 1966; Wersin et al, 1991)
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